Electro-pyrotechnic initiator built around a complete printed circuit

ABSTRACT

The invention relates to an initiator for a pyrotechnic gas generator intended for motor vehicle safety.  
     The initiator ( 2 ) is built from a circuit support ( 10 ) on which conductive strips are printed which include electromagnetic protection and the ends of which are joined together by a resistive element covered with a thermosensitive charge ( 6 ). The conductive strips are connected to an electric cable ( 3 ). The support ( 10 ) is coated in a cylindrical overmoulded insulation ( 4, 5 ).  
     A fragmentable cylindrical cap ( 8 ) containing an igniter composition ( 9 ) forms, together with the initiator ( 2 ), an igniter ( 1 ) for pyrotechnic generators intended for motor vehicle safety.

[0001] The present invention relates to the field of pyrotechnicinitiators and deals more specifically with an electro-pyrotechnicinitiator intended for motor vehicle safety.

[0002] Electro-pyrotechnic initiators are traditionally built from twoelectrically conducting pins which are held in place by overmouldedinsulation and the upper ends of which are joined together by a heatingresistive filament which is encased in an igniter bead consisting of aprimary explosive such as lead trinitroresorcinate or lead styphnate andthe lower ends of which are connected to a source of electrical current.

[0003] Such electro-pyrotechnic initiators are widely used to formdevices for igniting pyrotechnic gas generators intended to inflate airbags to protect the occupants of a motor vehicle. In order to protectthese initiators against parasitic electrostatic currents and avoiduntimely undesired and possibly dangerous ignition, a ferrite core isgenerally placed between the pins, as described, for example, in thePatents U.S. Pat. No. 4,722,551 and EP-A-0,512,682. In order further toimprove the ignition reliability, the heating resistive filament issometimes replaced by a printed circuit comprising a resistive andheating part as described, for example, in Patent ApplicationFR-A-2,704,944 or in its counterpart U.S. Pat. No. 5,544,585.

[0004] Whatever the case may be, systematically resorting to conductivepins appreciably increases the cost price and the complexity ofmanufacturing these electro-pyrotechnic initiators. What is more, thevery frequent use of ferrite cores increases the external dimensions ofthe overmoulding surrounding the pins and does nothing to help reducethe overall volume of the pyrotechnic gas generators intended for motorvehicle safety.

[0005] Those skilled in the art are therefore still looking forelectro-pyrotechnic initiators which are at the same time reliable intheir operation, easy and economical to manufacture and small in volume.

[0006] The object of the present invention is specifically to providesuch an initiator by dispensing with the pins and the ferrite cores andby integrating their functions into a complete printed circuit, contraryto all that has been done hitherto.

[0007] The invention therefore relates to an electro-pyrotechnicinitiator connected to an electric cable consisting of two conductivewires and comprising:

[0008] i) a thermosensitive charge that can be initiated by a heatingresistive element,

[0009] ii) an electric circuit connecting the said resistive element tothe said conductive wires,

[0010] iii) means of electromagnetically protecting the said electriccircuit,

[0011] characterized in that the said heating resistive element, thesaid electric circuit and the said means of electromagnetic protectionare integrated into a circuit of conductive strips which are soldered tothe said conductive wires and which are formed on a printed circuitsupport around which the said initiator is built.

[0012] The printed circuit support will normally consist of one of theinsulating substances conventionally used for making such supports.

[0013] According to a first preferred embodiment of the invention, thesaid heating resistive element itself consists of a resistive stripformed on the printed circuit support and the thermosensitive charge isdeposited on the said resistive strip.

[0014] The means of electromagnetically protecting the initiator againstparasitic currents may consist of at least one filtering coil placed onthe said circuit support. As a preference, in this case, the filteringcoil may itself be printed on the said circuit support and, togetherwith the electrical circuit, constitute a multi-layer printed circuit.

[0015] The said means of electromagnetic protection may alternativelyconsist of at least one capacitor arranged on the said circuit support.

[0016] According to a preferred embodiment of the invention, the saidelectric circuit and the said means of electromagnetic protection arecovered by a layer of insulating varnish.

[0017] The circuit support, which may or may not be covered with a layerof insulating varnish, will moreover be partially coated withovermoulded insulation so that only that part of the circuit supportthat carries the heating resistive element covered with thethermosensitive charge is left free. When the thermosensitive charge isnot itself at least partially held in place by the said overmoulding, itwill advantageously be protected by a fragmentable mask. A mask made ofvery thin metal or a mask made of plastic, for example of transparentplastic, are suitable.

[0018] According to a second preferred embodiment of the invention, thesaid circuit support is in the form of a thin parallelepipedal card withtwo opposite flat faces. This second embodiment of the invention makesit possible to build electro-pyrotechnic initiators in which a separatecircuit of conductive strips, each including a heating resistive elementcovered by a thermosensitive charge and means of electromagneticprotection is printed on each of the two flat faces of the circuitsupport. It is thus possible to build an initiator which, for a singleelectrical source, has two separate igniter heads and which isparticularly dependable and reliable.

[0019] The invention also relates to an electro-pyrotechnic igniter,characterized in that an initiator according to the invention issurrounded by a fragmentable cap containing a granular igniter powder.

[0020] The invention finally relates to the use of anelectro-pyrotechnic initiator according to the invention to ignite apyrotechnic generator of gas intended to inflate an air bag to protectthe occupant of a motor vehicle. The initiator may be used directly toignite the pyrotechnic charge of the generator when the latter is ingranular or pellet form, it will advantageously be used via an igniterlike the one described above when the charge is in the form of a compactblock.

[0021] The invention thus makes it possible, using small printedcircuits which are very simple and very economic to produce, toconstruct initiators or igniters which are not very bulky and are veryreliable.

[0022] A preferred embodiment of the invention will be describedhereinbelow with reference to FIGS. 1 to 8.

[0023]FIG. 1 is a view in perspective, with partial cutaway, of anigniter built from an initiator according to the invention depicted, forthe sake of clarity, without its protective mask.

[0024]FIGS. 2, 3 and 4 illustrate the successive stages in building aninitiator according to the invention starting with its circuit support.

[0025]FIG. 5 illustrates a printed circuit in which the electromagneticprotection is provided by capacitors.

[0026]FIG. 6 illustrates a printed circuit in which the electromagneticprotection is provided by a conventional filtering coil.

[0027]FIG. 7 illustrates a printed circuit in which the electromagneticprotection is provided by a coil printed in a multi-layer circuit.

[0028]FIG. 8 is a sectional view through an initiator comprising twoseparate igniter heads, the thermosensitive charges of which arepartially held in place by the overmoulding.

[0029]FIG. 1 depicts an igniter 1 consisting of an initiator 2 connectedto an electric cable 3.

[0030] Referring more specifically to FIGS. 1 and 4, it can be seen thatthe initiator 2 is contained for the most part in a cylindricalovermoulding 4 which at the same end as the cable 3 has a base 5 ofslightly larger diameter.

[0031] At its opposite end to the base 5, the initiator 2 has an igniterhead which will be described in detail a little later but which can beseen to have a thermosensitive charge 6 based on leadtrinitroresorcinate, which is protected by a mask 7 made of transparentplastic, not depicted in FIG. 1. A fragmentable metal cap 8 ofcylindrical shape encases the overmoulding 4, resting against the baseS. This cap is closed at its opposite end to the base 5 and contains apyrotechnic charge 9 which generates gas. A mixture of powder of theboron/potassium nitrate type, and of single-base nitrocellulose powderis ideal.

[0032] The construction of the initiator 2 will now be described indetail with reference more particularly to FIGS. 2 to 5. The initiator 2is built from a printed circuit support 10 which is in the form of athin parallelepipedal card with two opposite flat faces. This support 10is built from an epoxy resin filled with glass fibres. Printed on one ofits faces are two conductive strips 11 and 12, each of which is solderedat one of its ends to the conductive wires 13 and 14 that constitute theelectric cable 3. A heating resistive element 15 joins the free ends ofthe conductive strips 11 and 12 together. This heating resistive element15 may be a resistive wire but will advantageously consist of a heatingresistive strip defined by a printed sub-circuit 16 as described inPatent U.S. Pat. No. 5,544,585 or in its counterpart FR-A-2,704,944, thedescription of which is included, by reference, in this description.

[0033] The heating resistive element 15 could equally consist of a diodeor of a semiconductor bridge as described, for example, in PatentApplication FR-A-2,720,493.

[0034] Two capacitors 17 and 18 are arranged on and connected to thecircuit formed by the conductive strips 11 and 12 and by the resistiveelement 15. These capacitors constitute the means of electromagneticallyprotecting the circuit by preventing the discharge of parasitic currentsthrough the element 15.

[0035] These means of electromagnetic protection could also consist of afiltering coil 61 arranged on and connected to a circuit printed on asupport 60 and consisting of two conductive strips 62 and 63 joinedtogether by a resistive element 64 as depicted in FIG. 6. An alternativeembodiment of electromagnetic protection of this kind is depicted inFIG. 7. A circuit that consists of two conductive strips 72 and 73joined together by a resistive element 74 is printed on a support 70.The conductive strip 72 forms a coil 71 around the support 70, thusforming a multi-layer circuit with the strip 73.

[0036] Referring once again to FIG. 2, it can be seen that the part ofthe face of the support 10 on which the strips 11 and 12 are printed andon which the capacitors 17 and 18 rest is covered with a layer ofinsulating varnish 19. This layer 19 does not cover that end of thesupport 10 that carries the resistive element 15 and that willconstitute the igniter head of the initiator. Thus prepared, the support10 is partially surrounded by a cylindrical overmoulding 4 of adhesiveresin based on a hot-melt polyamide.

[0037] This overmoulding 4 leaves free that surface of the support 10which carries the resistive element 15, as depicted in FIG. 3. Thisresistive element 15, together with the sub-circuit 16 used to form itare covered with a thermosensitive charge 6, for example based on leadtrinitroresorcinate. The charge 6 is protected by a mask 7 made oftransparent plastic. The initiator 2 is thus finished and can be used toform the igniter 1 described earlier.

[0038]FIG. 8 depicts an initiator 82 which constitutes an alternativeform of the initiator 2 just described. The initiator 82 is built arounda circuit support 80 in the form of a thin parallelepipedal card. Theessential difference between this initiator 82 and the initiator 2described above lies in the fact that each of the two faces of thesupport 80 carries a complete printed circuit 87 or 89 connected to thesame cable 83 bringing in electrical current. The support 80 ispartially coated in a cylindrical overmoulding 84 which has a widenedbase 85 and which partially holds in place the thermosensitive charges86 and 88 that constitute the igniter heads. In this embodiment, theinitiator does not require a protective mask. Should current arrive viathe cable 83, the two thermosensitive charges are fired simultaneouslyand the initiator 82 is thus highly effective in normal operation andextremely safe, both with respect to parasitic currents—thanks to itsbuilt-in electromagnetic protection 91, 93 or 92, 94, and with respectto possible misfires—thanks to the fact that the heads are duplicated.

1. Electro-pyrotechnic initiator (2) connected to an electric cable (3)consisting of two conductive wires (13, 14) and comprising: i) athermosensitive charge (6) that can be initiated by a heating resistiveelement (15), ii) an electric circuit connecting the said resistiveelement (15) to the said conductive wires (13, 14), iii) means ofelectromagnetically protecting the said electric circuit, characterizedin that the said heating resistive element (15, 64, 74), the saidelectric circuit and the said means of electromagnetic protection areintegrated into a circuit of conductive strips (11, 12) which aresoldered to the said conductive wires (13, 14) and which are formed on aprinted circuit support (10, 60, 70, 80) around which the said initiatoris built.
 2. Initiator according to claim 1 , characterized in that thesaid heating resistive element consists of a resistive strip (16). 3.Initiator according to claim 2 , characterized in that the saidthermosensitive charge (6) is deposited on the said resistive strip(16).
 4. Initiator according to claim 3 , characterized in that the saidmeans of electromagnetic protection consist of at least one filteringcoil (61) placed on the said circuit support (60).
 5. Initiatoraccording to claim 3 , characterized in that the said means ofelectromagnetic protection consist of at least one filtering coil (71)printed on the said circuit support (70).
 6. Initiator according toclaim 3 , characterized in that the said means of electromagneticprotection consist of at least one capacitor (17, 18) arranged on thesaid circuit support (10).
 7. Initiator according to claim 3 ,characterized in that the said electric circuit and the said means ofelectromagnetic protection are covered by a layer of insulating varnish(19).
 8. Initiator according to either one of claims 3 and 7,characterized in that the said circuit support (10) is partially coatedwith overmoulded insulation (4).
 9. Initiator according to claim 8 ,characterized in that the said thermosensitive charge (6) is protectedby a fragmentable mask (7).
 10. Initiator according to claim 8 ,characterized in that the said circuit support (10) is in the form of athin parallelepipedal card with two opposite faces.
 11. Initiatoraccording to claim 10 , characterized in that a separate circuit (87,89) of conductive strips, each including a heating resistive elementcovered by a thermosensitive charge (86, 88) and means ofelectromagnetic protection (91, 93; 92, 94) is printed on each of thetwo flat faces of the circuit support (80).
 12. Electro-pyrotechnicigniter (1), characterized in that an initiator according to any one ofclaims 1 to 11 is surrounded by a fragmentable cap (8) containing anigniter powder (9).
 13. Use of an electro-pyrotechnic initiatoraccording to any one of claims 1 to 11 to ignite a pyrotechnic generatorof gas intended to inflate an air bag to protect the occupant of a motorvehicle.